Essentially, digital oscillators provide sequences of digitally-encoded values representing samples of continuous-time sinusoidal signals. The output sequences from digital oscillators can be called "sinusoidal digital sequences," "digitally-encoded sinusoidal signals," or "sampled sinusoidal signals" to distinguish them from analog or continuous-time sinusoidal signals produced, e.g., by analog oscillators.
Since the samples are typically taken at uniform intervals of time, the sampled sinusoidal signals from digital oscillators are characterized by sampling rates or frequencies ("f.sub.s ") in addition to being characterized by the frequencies ("f.sub.0 "), phases and amplitudes of the continuous sinusoids they specify.
With sufficient sample values for each cycle of the sinusoidal signal, i.e., with a sufficiently high ratio of f.sub.s to f.sub.0, the sinusoidal digital sequence can represent accurately a sinusoidal signal having any desired frequency, amplitude, and phase.
Known digital oscillators can be characterized as either signal-store-type or signal-generator-type oscillators. A signal-store-type digital oscillator can be implemented, for example, as a read-only-memory ("ROM"), in which pre-calculated digital sequences specifying one or more sinusoidal signals are stored. The ROM is accessed typically with the aid of addressing logic. This approach has the advantage that the stored digital sequences are readily and dependably available, and need not be generated each time they are needed in an application, and thus are not subject to errors that could be introduced during repeated generation.
On the other hand, the drawbacks of this approach are readily apparent. For instance, the size of the memory limits the number of stored digital sequences (and thus the number of sample values per cycle), or the number of sinusoidal signals specified by the stored sequences, or both.
On the contrary, signal-generator-type digital oscillators overcome many of these drawbacks since they avoid the required storage of sample values of sinusoidal signals, though at the expense of requiring arithmetic operations for generating the values each time they are needed.
A known version of such an oscillator uses registers and adders in a closed loop arrangement. The output of a first adder is fed to a first register, where it is stored for a clock cycle. Thereupon, that register passes its contents to both a second register and a multiplier. The second register stores the signal from the first register for one clock cycle, and then passes it to the adder as a first input. The multiplier forms the product of the signal from the first register and a coefficient "m" and provides the product to the adder as a second input. The adder forms the sum of the signal from the second register and the product from the multiplier, which sum is applied to the first register as described above. The second shift register also passes its contents onto an output line. The digital sequences on the output line are the samples of a sinusoidal signal of selected amplitude, phase and frequency.
While such signal-generator-type digital oscillators are generally adequate for many applications, they generally require relatively expensive, high-precision multipliers when sampling rates are high, e.g., above about 50 samples per cycle. On the other hand, when sample rates are low, e.g., below 20 samples per cycle, expensive low-pass filters typically are employed to smooth the resulting sinusoidal signals following conversion of the digital sequences from the oscillators to analog signals.
Also, such digital oscillators are designed to generate only a single sinusoidal sequence at a time. Accordingly, such digital oscillators are incapable of meeting the needs of certain telecommunication applications that require, e.g., concurrently-supplied plural sinusoidal signals of different (or even varying) amplitudes, phases or frequencies.
Consider an illustrative application that requires quadrature-related signals. Radio-telephone transmitters employing quadrature modulation, and receivers employing quadrature demodulation, each require simultaneously two sinusoidal signals that are exactly 90 degrees apart in phase, i.e., signals that are in perfect quadrature. Typically, in such an arrangement, the transmitter phase-modulates each of the quadrature-related signals with the information (e.g., voice) to be transmitted, and then combines the modulated signals for broadcast. Analogously, the receiver multiplies the received signals with quadrature-related signals to obtain separate "in-phase" and "quadrature" signals.
The accuracy of the recovery of the information can be affected detrimentally by any significant variation from perfect quadrature in the pairs of quadrature-related signals used in the transmitter and receiver.
Unfortunately, known signal-generator-type digital oscillators do not simultaneously supply perfectly quadrature signals for such applications in a reliable manner and without significant expense--both in terms of design and manufacturing costs and in terms of the number, size (i.e., real estate on an integrated chip) and power requirements of electronic components needed to implement such circuits.
Radio-telephones also use oscillator-supplied sinusoidal signals for other purposes, e.g., as system-test signals, and for supervisory-audio tones and dialing tones. Accordingly, radio-telephones require oscillators for producing a variety of different sinusoidal signals used for a variety of applications.